The present invention relates generally to methods for the removal of coatings from substrates and, more particularly, to a novel method for removing metallurgically bonded coatings from a substrate, such as a machine part, in which the coating may often be harder than the substrate.
In the manufacture of many machine parts, it is commonplace to apply a metallic or ceramic coating to improve the various physical characteristics of the part, such as surface hardness, heat resistance and wearability. For example, in the manufacture of aircraft, turbine engine, and pump and valve components, it is commonplace to apply a metallic and/or ceramic surface coating or coatings to the components, often in such a manner as to establish a diffusion or metallurgical bond between the component and the coating.
In turbine components, in particular, it is typical to apply a metallic coating, such as a so-called “MCrAlY” coating comprised of a metal (typically nickel or cobalt), chromium, aluminum, and yttrium, which provides advanced oxidation and corrosion resistance, which is often diffusion bonded to the surface of the component. In the hottest environments, a thermal barrier coating (TBC) outwardly bonded over the metallic coating is also applied.
When such components require repair or refurbishment, such coatings must be initially removed from the substrate component to which they are bonded. Since the coatings in many cases are harder than the material of the underlying component, the removal of the coatings presents the difficult problem of fully removing the coating without damaging the underlying component.
Several different techniques and methods are utilized conventionally to remove such coatings. In some cases, the coating may be mechanically machined or ground from the surface of the underlying component, but this technique is feasible essentially only as to components having a uniformly flat or curved (e.g., cylindrical) geometrical shape which has not become distorted during use, which is seldom the case with turbine engine components. An alternative removal technique is to subject the component surface to grit blasting, sometimes commonly referred to as sandblasting. Grit blasting accomplishes satisfactory results when the coating is softer or more brittle than the material of the underlying component, but is generally infeasible when the coating is harder than the underlying component. It is also known to use a high pressure water jet for stripping of coatings, but this process is effective only as to coatings which are only mechanically bonded to the underlying component, and is generally ineffective for removing diffusion or metallurgically bonded coatings. A chemical solution, ordinarily an acid solution, can also be utilized to remove a coating from an underlying base component, but such chemical stripping techniques pose environmental problems and also are generally effective only when a suitable chemical is available to preferentially attack the coating without damaging the underlying material of the base component.
Accordingly, a need exists for an improved methodology by which diffusion and other metallurgically bonded coatings may be removed from an underlying component, especially coatings which may be harder than the material of the underlying component or more resistant to stripping acid or other chemical stripping solution than the underlying component.